# Slicing simultaneity at an angle into past and furure

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According to this video 1:24:00). < hperlink .

If someone is a very long distance away and they reverse their direction of travel from away from me toward me, their experience of time is supposed to switch from my past to my future.

I have been trying to understand why their direction of travel makes this difference. How this is possible?

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Moderator Note

Moved to Relativity

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3 hours ago, bob campbell said:

If someone is a very long distance away and they reverse their direction of travel from away from me toward me, their experience of time is supposed to switch from my past to my future.

What do you mean by this? (pretend there’s no video; you have to summarize the argument here)

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3 hours ago, swansont said:

What do you mean by this? (pretend there’s no video; you have to summarize the argument here)

In summery I don't understand why time dilation would create a situation where reversing direction either toward or away from the observer would swing the one in motion from the observer's future into the observer's past.  The video was very explanatory, were you not able to watch it?

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9 minutes ago, bob campbell said:

The video was very explanatory, were you not able to watch it?

I watched a bit from where you linked. It made little sense to me. I suppose it might make more sense if I had watched the whole thing and so understood the model he was trying to explain. His explanation seems to be at the level of "this is what happens (trust me)". Maybe you need to watch his "more mathematical" video to understand.

Or find a better source. Personally, I find videos about the worst possible medium for learning and understanding things. I would prefer a good book or article. Why spend hours watching video, when you could read the same content in 30 minutes and it would have a clearer (hopefully) explanation.

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32 minutes ago, bob campbell said:

In summery I don't understand why time dilation would create a situation where reversing direction either toward or away from the observer would swing the one in motion from the observer's future into the observer's past.  The video was very explanatory, were you not able to watch it?

I’m nit required to watch it, by the rules, and it’s a 2.5 hour video, though it starts ~halfway through.

I watched a few seconds and he was talking about simultaneity, and the relative “positioning” of some value of t.

When you say “experience of time” being past or future, simultaneity is a big thing to leave out, if that’s what you want to know about. Is it? (if it is, it has to with how long a photon will take to get to you)

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2 minutes ago, swansont said:

I’m nit required to watch it, by the rules, and it’s a 2.5 hour video, though it starts ~halfway through.

I watched a few seconds and he was talking about simultaneity, and the relative “positioning” of some value of t.

When you say “experience of time” being past or future, simultaneity is a big thing to leave out, if that’s what you want to know about. Is it? (if it is, it has to with how long a photon will take to get to you)

I'm sorry this is my first question in this forum.  I did not know that I broke a rule and should not include a video link, even though it describes the problem I'm asking about with great clarity within the first minute.

I can see that you are having trouble understanding my question, so thank you for your attention, but I believe I found the answer in a http://galileo.phys.virginia.edu/classes/252/lorentztrans.html.

Specifically the cone of light section.

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35 minutes ago, bob campbell said:

I can see that you are having trouble understanding my question, so thank you for your attention, but I believe I found the answer in a http://galileo.phys.virginia.edu/classes/252/lorentztrans.html.

See, told you a written source would be better. 😃

36 minutes ago, bob campbell said:

a video link, even though it describes the problem I'm asking about with great clarity within the first minute.

If it described it with great clarity, you wouldn't have needed to ask the question 🤨

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1 hour ago, bob campbell said:

I'm sorry this is my first question in this forum.  I did not know that I broke a rule and should not include a video link, even though it describes the problem I'm asking about with great clarity within the first minute.

I watched that first minute, and what he's talking about is not well-captured in your question's phrasing.

In a single frame, all clocks can be synchronized, because you can adjust for the delay of a finite speed of light. Similarly, people in a frame can agree on when something happened by applying that delay. An event you observe that is 1 light-second away happened 1 second ago. Everybody in that frame agrees. Let's call that t=0

But moving changes that. If you are moving away, it takes longer for the light to get to you, because in that 1 second, you have moved some distance. So an identical event getting to you happened before t = 0, i.e. in the other frame's past (before t = 0), because an event that happened at t=0 can't reach you in 1 second.

Similarly, if you are moving toward the event, the light arrives faster, because you have reduced the distance.

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38 minutes ago, swansont said:

But moving changes that. If you are moving away, it takes longer for the light to get to you, because in that 1 second, you have moved some distance. So an identical event getting to you happened before t = 0, i.e. in the other frame's past (before t = 0), because an event that happened at t=0 can't reach you in 1 second.

Similarly, if you are moving toward the event, the light arrives faster, because you have reduced the distance.

Maybe Einstein's train / lightning thought experiment is helpful here: the lightning flashes at each end of the train are simultaneous in the frame of reference of an observer on the platform. For the passenger on the train, the one at the front of the train happens first (because they are moving towards it) while the one at the back happens later. So you can, I suppose, consider the platform observer seeing the passenger's "past" and "future" happening at the same time.

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5 hours ago, swansont said:

I watched that first minute, and what he's talking about is not well-captured in your question's phrasing.

In a single frame, all clocks can be synchronized, because you can adjust for the delay of a finite speed of light. Similarly, people in a frame can agree on when something happened by applying that delay. An event you observe that is 1 light-second away happened 1 second ago. Everybody in that frame agrees. Let's call that t=0

But moving changes that. If you are moving away, it takes longer for the light to get to you, because in that 1 second, you have moved some distance. So an identical event getting to you happened before t = 0, i.e. in the other frame's past (before t = 0), because an event that happened at t=0 can't reach you in 1 second.

Similarly, if you are moving toward the event, the light arrives faster, because you have reduced the distance.

That makes a lot of sense, and certainly agree. Light arrives faster, because you have reduced the distance. But what would cause the time frame to be sliced diagonally and swing from past to future by such a large amount into the past and future when the bicycle turns around?

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46 minutes ago, bob campbell said:

That makes a lot of sense, and certainly agree. Light arrives faster, because you have reduced the distance. But what would cause the time frame to be sliced diagonally and swing from past to future by such a large amount into the past and future when the bicycle turns around?

The bicycle rider has to measure the speed of light as being c, relative to himself, regardless of what direction he is traveling.

So let's say, that when he is ( by his measure) 1 light hr from a source that he has been moving away from at 0.5 c. That means that the light reaching him from that source at that moment had to leave the source when they were closer together, when they were only 2/3 of a light hour apart.  Thus it took the light just 2/3 of a hour to reach him, during which time the source traveled 1/3 of a light hour. to a distance of 1 light hour.

Further assume that that light carried an image of a clock at the source reading 12:00.  Since he knows that 2/3 of any hr passed since that image left, and that due to time dilation, The source clock runs 0.886 times as fast as his own,  the source clock should read between 12:34 and 12:35, at the moment he sees this light.

Now let's say that he is moving towards the source at 0.5c, when he sees the light while still 1 light from the source. Now he has to conclude that the source was further than 1 light hr away when the light left ( 2 light hrs away).  He still has to measure the light as traveling at c relative to himself, So, by his clock it took 2 hrs for the light to reach him, during which time he and the source closed to a distance of 1 light hr.  He sees the same image of 12:00 for the source clock*, But he now it has been two hrs since that image left the source, during which time the source clock advanced to read 13:44, which is the time on the source clock when he sees the image  This is quite a bit later than what he concluded when he was moving away from the clock.

So before the bicycle reverses direction, he would say that is 12:34 on the source clock, and afterwards he would say that it reads 13:44 ( even though the image he sees for the source clock remains 12:00)

For this we assume an instantaneous change in velocity, which can't actually happen.  The bicycle has to spent some non-zero time accelerating, during which period it is non-inertial.

It is a bit more complicated to work out what someone concludes when working from a non-inertial frame.  For one thing, it is only the Local proper speed of light that is measured as being c.  The "coordinate" speed of light changes with distance from the observer.  So said observer cannot conclude that the light traveling from the source traveled at c relative to him for the entire trip.  The upshot is that for this accelerating observer, clocks in the direction of the acceleration, run fast by a rate that depends on the magnitude of the acceleration and the distance to the clock.   This leads our bike rider to conclude that while he decelerates to a stop and then accelerates back up to speed back towards the source, the source clock runs very fast and ticks off the time between  12:34-12:35 and 13:44, over a very short period by the bike rider's clock.

* If it helps, assume two bicycles, one moving towards and one moving away from the source and passing each other when they are 1 light year away from the source.  They both need to see the same image of the source at that moment.

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I'm very sorry that I have not been able to describe clearly the problem.  I fear your wrath by suggesting that you watch the video from 1:18:30

But I am using the video because as they say a picture is worth a thousand words, and this is difficult to describe.

I understand time dilation but this time difference is far more than simple time dilation can account for in a situation where relative speed difference is so small.

I will attempt once again to ask my question with words.

According to this video, if someone is a very long distance away and they reverse their direction of travel from away from me to toward me, their experience of time is supposed to switch from my past to my future.

I have been trying to understand why their direction of travel makes this much difference. How this is possible to switch from past to future. I get that there is a very small change in time dilation, but it seems that the Lorentz factor would have to switch from positive to negative and even if that happened why would the amount of time vary by so much between past and future?  It has something to do with the very long distances.  If you chose to watch the video you should only have to watch from 1:18:30  till about 1:27:00 to understand exactly what I'm asking.

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9 hours ago, bob campbell said:

But I am using the video because as they say a picture is worth a thousand words, and this is difficult to describe.

The 600 words posted by Janus are worth more than a couple of hours of video

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17 hours ago, bob campbell said:

I have been trying to understand why their direction of travel makes this much difference. How this is possible to switch from past to future. I get that there is a very small change in time dilation, but it seems that the Lorentz factor would have to switch from positive to negative and even if that happened why would the amount of time vary by so much between past and future?  It has something to do with the very long distances.  If you chose to watch the video you should only have to watch from 1:18:30  till about 1:27:00 to understand exactly what I'm asking.

Yes, you're correct that it's about distance. The effect still occurs at low speed where time dilation is negligible. Google "Andromeda paradox", the effect can happen at walking speeds if the distance is large enough.

The reason direction matters is that if the distant location is far enough, the travel time of light is long enough that you can move a significant distance even at slow speeds. If a source is millions of light years away, you can walk on the order of light days between "now" and when light arrives. Time dilation still applies but with vanishing speed it approaches zero. At walking speeds it might contribute seconds where light travel time contributes days.

Assuming a time dilation factor rounded to 1, suppose two people separated by two light days walked toward each other, and meet after a million years, at which point they both receive a signal from a source that is "now" 1 million light years away. The one walking away from it says "the source is moving away from me and this light took one million years minus a day to reach me; the signal was sent after we started walking" and the other says "this same light took one million years plus a day; it was sent before we started walking" and they disagree on what is "now" at the source just like they did when they started walking. That is pretty much their "experience" of the phenomena.

The virginia.edu link mentions a causal definition of past/future:

Quote

Possible causal connections are as follows: an event at the origin (0, 0, 0, 0) could cause an event inside or on the forward light cone: so that is the “future”, as seen from the origin.  Events in the backward light cone—the “past”—could cause an event at the origin. There can be no causal link between an event at the origin and an event outside the light cones, since the separation is spacelike: outside the light cones is “elsewhere” as viewed from the origin.

When you're talking about "switching between past and future", you're talking about events that are in each other's elsewhere... it's switching from one part of the elsewhere to another.  "Now" far away is neither in the causal past nor future of "now" here. The two observers moving in different directions only "experience" such a switch after a million years in this example, involving measurements made when they're far apart from each other.

Edit: That last part's misleading, the observers don't have to be separated. Rather... direction matters because the distant object is moving in different directions relative to the two observers, so the observers disagree on how far the same light signal from it has travelled.

Edited by md65536
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3 hours ago, md65536 said:

10 hours ago, Strange said:

😍 md65536 Thank you for the name of this problem!  The Andromeda Paradox.  I was finally able to find the math proof that I had been looking for.  You're the best!  Isn't this a fun paradox to think about?

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On 8/4/2020 at 11:21 AM, Janus said:

....

Thanks. That made a few things really click in my head, in regards to understanding the "twins paradox".

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On 8/3/2020 at 5:21 PM, Janus said:

The upshot is that for this accelerating observer, clocks in the direction of the acceleration, run fast by a rate that depends on the magnitude of the acceleration and the distance to the clock.   This leads our bike rider to conclude that while he decelerates to a stop and then accelerates back up to speed back towards the source, the source clock runs very fast and ticks off the time between  12:34-12:35 and 13:44, over a very short period by the bike rider's clock.

Are you okay with saying that the source clock runs backward if/while the very distant rider accelerates away from it? If that's not okay then what you wrote could be phrased differently to avoid it (eg. relative time instead of how the clock itself runs).

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On 8/3/2020 at 2:09 AM, bob campbell said:

According to this video 1:24:00). < hperlink .

If someone is a very long distance away and they reverse their direction of travel from away from me toward me, their experience of time is supposed to switch from my past to my future.

I have been trying to understand why their direction of travel makes this difference. How this is possible?

He's talking about time dilation. If you have a relativistic object events outside of that object occur more quickly so that object is sort of stuck in the past. Now reversing direction at those extreme relativistic speeds length contraction reverses so the object experiences length dilation and time contraction so time relative to that object occurs much faster than outside that object so long as it is decelerating from relativistic speeds.

That's actually a proposed way for a spacecraft to buy back time lost due to time dilation

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1 hour ago, IDoNotCare said:

He's talking about time dilation. If you have a relativistic object events outside of that object occur more quickly so that object is sort of stuck in the past. Now reversing direction at those extreme relativistic speeds length contraction reverses so the object experiences length dilation and time contraction so time relative to that object occurs much faster than outside that object so long as it is decelerating from relativistic speeds.

You couldn't be be more wrong if your name were Wrongy McWrongface and you posted this in the wrong forum on the wrongest day of the year after passing the text through an electric wrongifier.

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10 hours ago, md65536 said:

Are you okay with saying that the source clock runs backward if/while the very distant rider accelerates away from it? If that's not okay then what you wrote could be phrased differently to avoid it (eg. relative time instead of how the clock itself runs).

The source clock runs slower (not backwards), if you are accelerating away from it( by your measure).  The rate does depend on the distance. The further away, the slower the clock runs.*

Now, for any acceleration value there is a distance in that direction where the coordinate speed of light goes to zero.  This results in a Rindler horizon beyond which the accelerating observer cannot make any measurement.  Light originating from beyond the Rindler horizon will not reach you.  This is true even if the source of that light shares the observers acceleration.

* this in addition to any time dilation due to a difference in velocity.

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On 8/3/2020 at 5:21 PM, Janus said:

This leads our bike rider to conclude that while he decelerates to a stop and then accelerates back up to speed back towards the source, the source clock runs very fast and ticks off the time between  12:34-12:35 and 13:44, over a very short period by the bike rider's clock.

5 minutes ago, Janus said:

The source clock runs slower (not backwards), if you are accelerating away from it( by your measure).

These two statements aren't consistent with each other.

If the bike rider accelerates, ie. changes inertial frame, there is a shift in relative simultaneity with respect to the distant source clock. If the rider accelerates towards it, the coordinate time of the distant clock can change from 12:34 to 13:44 in a small local time, as in your example. But if the bike rider then accelerates away and returns to its former reference frame, the coordinate time can change from 13:44 to 12:34, or even earlier if it accelerates away more. If the first change in relative simultaneity is "the source clock runs very fast", how is the second not running backwards?

It's because of confusion like this that I don't like the phrasing that a change in relative simultaneity means a clock is running fast, because it also implies a clock can run backwards. They're both just changes in relative simultaneity, but one is accepted as intuitively reasonable but the other isn't, meaning it isn't an intuitive way to describe it.

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